<!DOCTYPE html >
<html xmlns="http://www.w3.org/1999/xhtml">
<head lang="en">
    <meta charset="UTF-8" http-equiv="X-UA-Compatible" content="IE=Edge"/>
    <title>hello</title>
    <style>
        .duiqi{
            width: 500px;
            height: 400px;
            border-style: solid;
            font-size: 2rem ;
            line-height: 2rem;
            /*text-align: center;*/
        }
        .xiabiao{
            font-size: 60% ;
            vertical-align: -.2px;
        }
    </style>

</head>

<body>
<div class="duiqi">
    对齐H<span class="xiabiao">2</span>O
</div>
</body>
</html>